1. Field of the Invention
The present invention relates to surface relief diffractive optical elements, and in particular to surface relief diffractive optical elements having diffraction structure with a refractive index variation distribution layer.
2. Description of the Invention
Typical diffractive optical elements or grating elements can be classified into two types as follows. One type is volume diffractive optical elements in which three kinds of exposure materials are used. The first kind of exposure material is photopolymer which has the advantages of high diffraction efficiency and some even no need to use chemical agents for developing and fixing, but its high cost results in being not ready for cost effective production even high-volume replication process exists. The second kind of exposure material is silver halide emulsions which has high photo-sensitivity and makes photographing simpler than other recording materials, but it suffers from low diffraction efficiency, if no bleaching, and is vulnerable to humid. The third kind of exposure material is dichromate gelatin (DCG) which has relatively high diffraction efficiency, but also suffer from the same humidity problem as silver halide emulsions. Therefore, these three kinds of exposure materials are not suitable for cost effectively mass production of high efficiency diffractive optical elements. Surface relief type diffractive optics is the most common choice for mass production of diffractive optical elements and/or gratings. A quartz, glass, silica or semiconductor material in the forms of thin film or substrate into which a fringe pattern of computer generated hologram(CGH) was fabricated by photolithography and followed by dry ion beam etching or chemical etching process. Another fabrication method of surface relief diffractive optical elements is using photoresist as exposure material and directly interfering with two laser beams. That is, a substrate coated with a photoresist layer, similar to photolithography process, is subject to a laser beam exposure process and forming interference fringe pattern onto the photoresist layer. Thereafter, fringe pattern was inscribed into substrate by a dry and/or wet etching process. In order to form a nickel mandrel or mode for high-volume production, the complete fabricated substrate, coated with a thin conducting metal layer, e.g., Ag, was immersed in electroplating solution for Ni electroforming. The mode or mandrel mentioned above can be used for plastic injection molding, embossing and stamping, etc., and has the advantages of ease mass production and low cost. Moreover, the surface relief diffractive optics is more durable in high humidity environment than the volume types. However, surface relief diffractive optics usually suffer from lower diffraction efficiency, when optical systems require each optical components should meet their presumed high enough light modulation efficiency. For diffractive optical elements, the light modulation efficiency, i.e., diffraction efficiency, is depending on the fringe cross-section profile of interference fringe pattern. In words, high diffraction efficiency is the main target for most diffractive optical elements application cases. According to theoretical analysis, a blazed or high aspect ratio grating and/or etch profile can be more efficient than other profiles. Practically, by wet or dry ion beam etching, the above factor is hard to control and be achieved during fabrication process. For wet etching, the pitch undercut (over-etching) phenomena always destroys the fine grating/fringe pitches. On the other hand, an etch profile, with high aspect ratio, of high-resolution interference fringes is also difficult to be obtained by dry ion beam etching. Therefore, various methods of fabricating blazed grating or diffractive optical elements according to the prior art are disclosed. For example, a blazed grating element can be produced by a staircase structure approach, which uses multi-step mask alignment, exposure and dry etching process. As shown in FIG. 1, a to near blazed multi-level grating profile structure 20 is formed on a quartz or silicon substrate 10 by a photolithography technique using multi-steps electron beam photo masks and dry ion beam anisotropic etching technique. Alternatively, as shown in FIG. 2, a blazed grating structure 40 is formed on a quartz or silicon substrate 30 by a gray-tone mask 50 lithography technique to create blazed photoresist grating layer, then, and etch into substrate. The above-mentioned gray-tone exposure process can also be replaced with a laser or electron beam direct writing process to perform the multi-steps exposure, although laser writing system sometimes does not have enough resolution to write a fringe pattern. In the methods mentioned above, by controlling the grating/etch profile shape, enhancement of first-order diffraction and reducing or eliminating unnecessary diffraction orders are achieved. Highly expensive gray-tone mask, electron beam and multi-steps masks make the fabrication process much more costly, moreover, the processing difficulty arising from the precise alignment and positioning become troublesome.
In the prior art of U.S. Pat. No. 4,426,130, a layer of transparent material having effectively two serially spaced sinusoidal phase gratings of the same line spacing, each formed as a surface relief pattern. Practically, by means of thin film coating technique, it is hard to make such a structure to superpose two or more gratings precisely in series thereon. In view of the above, to resolve the above-encountered problems, the object of the invention is to provide a method for manufacturing high-efficiency surface relief diffractive optical elements by using a grating material layer with continuous or discrete, i.e., multi-layer, refractive index variation distribution which was obtained by a refractive index controlled coating and, thereafter, a dry ion beam anisotropic etching process. Therefore, surface relief diffraction elements having high efficiency can be created by the conventional thin film coating and dry etching technologies without the processes of using costly electron beam masks or gray tone masks as well as multi-level lithography which uses repetitively precise mask positioning and aligning techniques.
In principle, a phase grating or diffractive optical elements with surface relief structure is formed on a transparent substrate by a dry etching process. A diffraction phenomenon due to optical path difference is created when a light beam passes through the surface relief structure of the substrate. To surface relief diffractive optical elements or grating, the diffraction efficiency, the ratio of modulated light intensity vs. incident beam intensity, depends on the optical path difference between incident beam and outgoing beam; for example, the grating/etch profile aspect ratio. That is, once the modulation specification of the diffractive optical elements is determined, the interference fringe pattern and optimized grating aspect ratio are also determined. Since an optimized grating aspect ratio means that the sufficient optical path difference can be provided, in order to obtain high diffraction efficiency, how to manufacture surface relief diffractive optical elements having sufficient deep or blazed grating is a very important issue. Since the optical path difference is a function of light path and refractive index of grating material, therefore, control of optical path difference may be achieved by alternatively changing refractive index. In general, a thin film coating process can accomplish this alternative method.
Furthermore, the surface relief structure formed into a quartz or silicon substrate by a photolithography and dry etching process is basically a square or rectangular profile. If the exposure process is performed directly by two laser beam interference, a sinusoidal structure will be formed. However, from the fabrication point of view, it is difficult to form grating profile other than the above two structure types.
The invention combines the above-described surface relief phase grating or diffractive optical elements and gradient refractive index technology to manufacture diffractive optics with high diffraction efficiency. Since, unlike the prior art, it is not necessary to shape the grating profile and/or the aspect ratio of the surface relief structure in order to control the optical path difference of incident light beams, thereby the process becomes difficult and complicated. The present invention has the advantages of making mask alignment easy and suitable for high-volume production, and low cost.
First, a gradient refractive index layer is formed on a substrate by thin film coating technology. This can be only a single layer with continuously gradient refractive index distribution or composed of many discrete layers in which the gradient refractive index is controlled layer by layer. The types of gradient refractive index distribution are categorized into gradually increasing, gradually decreasing and a distribution function. These types of distributions can be analyzed by the rigorous coupled wave theory of diffraction analysis basing on the Maxwell Equations. The rigorous coupled wave theory decomposed the grating layer into many sub-layers and assumed that the incident wave is diffracted into several diffraction orders in each sub-layer with their own refractive indices. A layer-by-layer analysis was done in order to calculate the final intensity of the outgoing light beam. Incorporating with the refractive index of each sub-layer and the boundary condition. The rigorous coupled wave theory provides required diffraction efficiency estimation. Then, the gradient (continuous or discrete/multi-layer) refractive index layer formed on the substrate is designed with layers of different material or composition to meet the required efficiency. A real-time monitoring on the thin film coating process is used so that the gradient refractive index layer is obtained layer by layer with the designed refractive index distribution. Thereafter, the substrate with the gradient index layer is coated with a photoresist for interference fringe pattern transfer. After that, a required interference fringe pattern is formed on the photoresist by lithography using a mask or by laser interference. Finally, the fringe pattern is formed on the substrate by a conventional dry ion beam etching process. As described above, the high-efficiency surface relief diffractive optics is manufactured by etching the substrate on which the gradient refractive index distribution layer is formed without the steps of using costly electron beam masks or gray-tone masks as well as repetitive precise mask positioning and aligning technique.